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%I #13 Jan 14 2023 09:39:49
%S 1,4,28,232,2092,19864,195352,1970896,20275692,211825600,2240855128,
%T 23952786400,258287602744,2806152315048,30686462795856,
%U 337490492639512,3730522624066540,41422293291178872,461802091590831904,5167329622166765872
%N Cogrowth sequence of the Brin-Navas group B.
%C This is the number of words of length 2n equal to the identity in the letters t, t^{-1}, a, a^{-1} which generate the Brin-Navas group B.
%H Andrew Elvey Price, <a href="/A359705/b359705.txt">Table of n, a(n) for n = 0..126</a>
%H Andrew Elvey Price and A. J. Guttmann, <a href="http://arxiv.org/abs/1706.07571">Numerical studies of Thompson's group F and related groups</a>, arXiv:1706.07571 [math.GR], 2017.
%e For n=1 there are 4 words of length 2 equal to the identity: aa^{-1}, a^{-1}a, tt^{-1}, t^{-1}t.
%K nonn,walk
%O 0,2
%A _Andrew Elvey Price_, Jan 11 2023